Processor Speed Deterioration

Does the processor speed of a computer deteriorate over time, I have a 2GHz Core2 Duo laptop and My computer properties used to say 2.00 GHz and now it says 1.99 GHz, not a big difference but this program http://www.wizard-soft.com/cpuspeed/index.htm reports my speed as 1963.9 MHz and I8KFANGUI Reports my speed as 1997 to 2000